Pillar devices and methods of making thereof

ABSTRACT

A method of making a semiconductor device includes providing an insulating layer containing a plurality of openings, forming a first semiconductor layer in the plurality of openings in the insulating layer and over the insulating layer, and removing a first portion of the first semiconductor layer, such that first conductivity type second portions of the first semiconductor layer remain in lower portions of the plurality of openings in the insulating layer, and upper portions of the plurality of openings in the insulating layer remain unfilled. The method also includes forming a second semiconductor layer in the upper portions of the plurality of openings in the insulating layer and over the insulating layer, and removing a first portion of the second semiconductor layer located over the insulating layer. The second conductivity type second portions of the second semiconductor layer remain in upper portions of the plurality of openings in the insulating layer to form a plurality of pillar shaped diodes in the plurality of openings.

The present invention relates generally to the field of semiconductordevice processing, and specifically to pillar devices and a method ofmaking such devices.

BACKGROUND

Herner et al., U.S. patent application Ser. No. 10/955,549 filed Sep.29, 2004 (which corresponds to US Published Application 2005/0052915A1), hereby incorporated by reference, describes a three dimensionalmemory array in which the data state of a memory cell is stored in theresistivity state of the polycrystalline semiconductor material of apillar shaped semiconductor junction diode. A subtractive method is usedto fabricate such pillar diode devices. This method includes depositingone or more silicon, germanium or other semiconductor material layers.The deposited semiconductor layer or layers are then etched to obtainsemiconductor pillars. A SiO₂ layer can be used as a hard mask for thepillar etching and removed afterwards. Next, SiO₂ or other gap filldielectric material is deposited in between and on top of the pillars. Achemical mechanical polishing (CMP) or etchback step is then conductedto planarize the gap fill dielectric with the upper surface of thepillars.

For additional description of the subtractive pillar fabricationprocess, see Herner et al., U.S. patent application Ser. No. 11/015,824,“Nonvolatile Memory Cell Comprising a Reduced Height Vertical Diode,”filed Dec. 17, 2004 and U.S. patent application Ser. No. 11/819,078filed Jul. 25, 2007.

However, in the subtractive method, for small diameter or width pillartype devices, care must be taken to avoid undercutting the pillar at itsbase during the etching step. Undercut pillar devices may be susceptibleto falling over during subsequent processing. Furthermore, for smallerpillar devices, the height of the semiconductor pillar may be limited bythin and soft photoresist used as the etching mask, the oxide gapfilling step presents a processing challenge when the aspect ratio ofthe openings between the pillars increases, and the CMP process oretchback of the gap fill layer may remove a significant thickness of thedeposited semiconductor material.

SUMMARY

One embodiment of this invention provides a method of making asemiconductor device, which includes providing an insulating layercontaining a plurality of openings and forming a first semiconductorlayer in the plurality of openings in the insulating layer and over theinsulating layer. The method also includes removing a first portion ofthe first semiconductor layer, such that first conductivity type secondportions of the first semiconductor layer remain in lower portions ofthe plurality of openings in the insulating layer, and upper portions ofthe plurality of openings in the insulating layer remain unfilled. Themethod also includes forming a second semiconductor layer in the upperportions of the plurality of openings in the insulating layer and overthe insulating layer, and removing a first portion of the secondsemiconductor layer located over the insulating layer. The secondconductivity type second portions of the second semiconductor layerremain in upper portions of the plurality of openings in the insulatinglayer to form a plurality of pillar shaped diodes in the plurality ofopenings.

Another embodiment provides a method of making a semiconductor device,comprising forming a plurality of tungsten electrodes, nitriding thetungsten electrodes to form tungsten nitride barriers on the pluralityof tungsten electrodes, forming an insulating layer comprising aplurality of openings such that the tungsten nitride barriers areexposed in the plurality of openings in the insulating layer, andforming a plurality of semiconductor devices on the tungsten nitridebarriers in the plurality of openings in the insulating layer.

Another embodiment provides a method of making a semiconductor device,comprising forming a plurality of tungsten electrodes, selectivelyforming a plurality of conductive barriers on exposed upper surfaces ofthe tungsten electrodes, forming an insulating layer comprising aplurality of openings such that the plurality of conductive barriers areexposed in the plurality of openings in the insulating layer, andforming a plurality of semiconductor devices on the conductive barriersin the plurality of openings.

Another embodiment provides a method of making a semiconductor device,comprising forming a plurality of lower electrodes over a substrate,forming an insulating layer containing a plurality of first openingshaving a first width, such that the lower electrodes are exposed in thefirst openings, forming first semiconductor regions of a firstconductivity type in the first openings, forming a sacrificial materialin the plurality of first openings over the first semiconductor regions,forming a plurality of second openings in the insulating layer to exposethe sacrificial material, the second openings having a second widthgreater than the first width, removing the sacrificial material from thefirst openings through the second openings, forming second semiconductorregions of a second conductivity type in the first openings, wherein thefirst and the second semiconductor regions form pillar shaped diodes inthe first openings, and forming upper electrodes in the second openingsin the insulating layer such that the upper electrodes contact thesecond semiconductor regions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1C and 1E are side cross-sectional views illustrating stagesin formation of a pillar device according to the first embodiment of thepresent invention. FIGS. 1B and 1D are three dimensional views of thestages shown in FIGS. 1A and 1C, respectively.

FIGS. 2A to 2C are side cross-sectional views illustrating stages information of a pillar device according to the second embodiment of thepresent invention.

FIGS. 3A to 3E are side cross-sectional views illustrating stages information of a pillar device according to the third embodiment of thepresent invention.

FIGS. 3F and 3G are micrographs of exemplary devices made according tothe third embodiment.

FIG. 4 is a three dimensional view of a completed pillar deviceaccording to one or more embodiments of the present invention.

FIG. 5A is a prior art plot of etch rate versus polysilicon doping.FIGS. 5B to 5E are side cross-sectional views illustrating stages information of a pillar device according to the fourth embodiment of thepresent invention.

FIGS. 6A to 6G are side cross-sectional views illustrating stages information of a pillar device according to the fifth embodiment of thepresent invention.

FIGS. 7A and 7B are side cross-sectional views of device features madeaccording to the embodiments of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The present inventors realized that for semiconductor pillar deviceshaving at least two different conductivity type regions, such as a diodecontaining both p-type and n-type semiconductor regions, special stepshave to be taken to avoid shorting such a device when the device isformed in an opening in an insulating layer.

For example, if the conductive barrier layer is simply deposited intothe opening and then planarized, then the conductive barrier layer willextend along the sidewalls of the opening from the bottom to the top ofthe opening. If a semiconductor diode is then deposited into theopening, then the conductive barrier layer located along the sidewallsof the opening would short the p-type region of the diode to the n-typeregion of the diode.

Furthermore, if the semiconductor layers of the diode are formed by amethod such as low pressure chemical vapor deposition (LPCVD), then theconformal deposition fills the opening from sides, not exclusively fromthe bottom. Thus, if the n-type semiconductor is deposited in theopening first, then it would either also be located along the entiresidewalls of the opening or it would fill the entire opening. If then-type region is located along the sidewalls of the opening and thep-type region is located in the middle of the opening, then the upperelectrode would contact both the p-type and the n-type regions. If then-type region fills the entire opening, then there would be no place toform the p-type region in the opening to form the diode.

The embodiments of the present invention provide methods to overcomethese problems. In the first embodiment, the barrier layer isselectively formed to avoid shorting the diode formed in the opening inthe insulating layer above the barrier. In a first aspect of the firstembodiment, the barrier layer may be formed by nitriding the underlyingtungsten electrode to form a tungsten nitride barrier layer before orafter forming the insulating layer. If the tungsten nitride barrier isformed after forming the insulating layer, then the barrier layer isformed by nitriding a portion of the tungsten electrode exposed in theopening in the insulating layer. This step of nitriding through theopening in the insulating layer is used to selectively form a tungstennitride barrier layer on the bottom of the opening. In an alternativeaspect of the first embodiment, the barrier layer is formed bynitridation on the electrode prior to formation of the insulating layer.

In the second embodiment, the barrier layer is formed by selectivedeposition on the underlying electrode. In the third embodiment, aselective silicon recess etch that can be precisely controlled is usedto recess a silicon layer of one conductivity type in the opening priorto forming a silicon layer of the opposite conductivity type in thespace in the opening created by the recess etch.

FIGS. 1 and 2 illustrate methods of making a nitrided barrier layeraccording to alternative aspects of the first embodiment. FIGS. 1A and1B show a side cross sectional view and a three dimensional view,respectively, of a plurality of conductive electrodes 1 separated fromeach other by an insulating material or layer 3. The electrodes may haveany suitable thickness, such as about 200 nm to about 400 nm. Theelectrodes 1 may comprise tungsten or another conductive material thatcan be nitrided. The insulating material may comprise any suitableinsulating material, such as silicon oxide, silicon nitride, a highdielectric constant insulating material, such as aluminum oxide,tantalum pentoxide, or an organic insulating material. The electrodesmay be formed by depositing a tungsten layer over any suitablesubstrate, photolithographically patterning the tungsten layer intoelectrodes 1, depositing an insulating layer over and between theelectrodes 1, and planarizing the insulating layer by chemicalmechanical polishing (CMP) or etchback to form the insulating materialregions 3 which isolate the electrodes 1 from each other. Alternatively,the electrodes 1 may be formed by a damascene method, in which groovesare formed in the insulating layer 3, a tungsten layer is formed in thegrooves and over the upper surface of the insulating layer 3, followedplanarization of the tungsten layer by CMP or etchback to leave theelectrodes 1 in the grooves in the insulating layer 3. The electrodes 1may be rail shaped electrodes as shown in FIG. 1B. Other electrode 1shapes may also be used.

FIGS. 1C and 1D illustrate a step of nitriding the tungsten electrodes 1to form tungsten nitride barriers 5 on the plurality of tungstenelectrodes before the damascene type insulating layer is deposited onthe electrodes 1. The barriers 5 may have any suitable thickness, suchas about 1 nm to about 30 nm for example. Any nitriding method may beused. For example, a plasma nitriding method may be used in which anitrogen containing plasma, such as an ammonia or nitrogen plasma, isprovided to the surface of coexposed tungsten 1 and dielectric 3. Thespecifics of an exemplary plasma nitridation of tungsten to formtungsten nitride is described in U.S. Pat. No. 5,780,908, which isincorporated herein by reference in its entirety. It should be notedthat the method in U.S. Pat. No. 5,780,908 is used to form a nitridedtungsten surface to provide a barrier between the tungsten and analuminum layer above it, for the purpose of forming a metal gate, ratherthan for forming a barrier below a semiconductor device.

While tungsten was described as being used as the electrode 1 material,other materials, such as titanium, tungsten silicide or aluminum mayalso be used. For example, the stability of the tungsten nitride layerformed by nitridation of a tungsten silicide surface is discussed inU.S. Pat. No. 6,133,149 which is incorporated herein by referenced inits entirety.

The plasma nitridation nitrides the entire exposed surfaces of theelectrodes 1 and insulating layer 3. This leaves a surface which is parttungsten nitride barriers 5 and part nitrogen containing insulatingmaterial 7 portions. For example, if the insulating material 3 wassilicon oxide, then its upper portion is converted to silicon oxynitride7 after the nitridation. Of course if the original insulating material 3was silicon nitride, then the nitridation may form a nitrogen richsilicon nitride region 7 in the upper portion or surface of insulatingmaterial 3. Thus, the upper portions of the insulating layer or material3 which separates adjacent tungsten electrodes 1 from each other is alsonitrided during the nitriding step.

As shown in FIG. 1E, a second insulating layer 9 is deposited over thetungsten nitride barriers 5 and over the nitrided insulating material 7.The insulating layer 9 may have a better adhesion to the tungstennitride surface than to an unnitrided tungsten surface. The insulatinglayer 9 may comprise any suitable insulating material, such as siliconoxide, silicon nitride, a high dielectric constant insulating material,such as aluminum oxide, tantalum pentoxide, or an organic insulatingmaterial. The material of layer 9 may be the same as or different fromthe material of insulating layer 3.

A plurality of openings 11 are formed in the insulating layer 9 suchthat the tungsten nitride barriers 5 are exposed in the plurality ofopenings 11. The openings 11 may be formed by photolithographicpatterning, such as by forming a photoresist layer over the insulatinglayer 9, exposing and developing (i.e., patterning) the photoresistlayer, etching the openings 11 in layer 9 using the photoresist patternas a mask, and removing the photoresist pattern.

Thus, in the method of FIGS. 1A-1D, the step of nitriding to form thebarriers 5 occurs before the step of forming the insulating layer 9. Theinsulating layer 9 is formed on the tungsten nitride barriers 5 followedby forming the plurality of openings 11 in the insulating layer 9 toexpose upper surfaces of the tungsten nitride barriers 5.

A plurality of semiconductor devices are then formed on the tungstennitride barriers 5 in the plurality of openings 11 in the insulatinglayer 9. For example, a silicon layer 13, such as a doped polysilicon oramorphous silicon layer is deposited on the barriers 5 in the openings11. The formation of the semiconductor devices, such as pillar shapeddiodes, will be described in more detail with respect to the thirdthrough fifth embodiments below.

FIGS. 2A-2C illustrate an alternative method of the first embodiment inwhich the insulating layer 9 is formed on the plurality of tungstenelectrodes 1 (and on the insulating material or layer 3) before theformation of the barriers 5. A plurality of openings 11 are then formedin the insulating layer 9 to expose the upper surfaces of the pluralityof tungsten electrodes 1 as shown in FIG. 2A. As shown in FIG. 2B, thestep of nitriding occurs after the step of forming the plurality ofopenings 11 in the insulating layer 9 such that upper surfaces of theplurality of tungsten electrodes 1 are nitrided through the plurality ofopenings 11. For example, as shown in FIG. 2B, the nitrogen containingplasma 15 is provided into the openings 11 to nitride the tungstenelectrodes 1. The nitridation forms the tungsten barriers 5 on thetungsten electrodes 1 in the openings 11.

Thus, the nitriding step is performed after forming the plurality ofopenings 11 in the insulating layer 9 to form the tungsten nitridebarriers. Optionally, the nitriding step also nitrides at least onesidewall 12 of the plurality of openings 11 in the insulating layer 9.If the insulating layer 9 is silicon oxide, then the sidewalls 12 willbe converted to a silicon oxynitride region 14. As used herein, the term“sidewalls” will refer to both one sidewall of an opening having acircular or oval cross section or to plural sidewalls of an openinghaving a polygonal cross section for convenience. Thus, the use of theterm “sidewalls” should not be interpreted as being limited to sidewallsof an opening with a polygonal cross section. If the insulating layer 9is a material other than silicon oxide, then it may also be nitrided.For example, metal oxides may also be converted to a metal oxynitride,silicon nitride may be converted to a nitrogen rich silicon nitride,while organic materials will contain a nitrogen rich region 14.

FIG. 2C shows the formation of the silicon layer 13 in the openings 11.Details of layer 13 deposition will be provided with respect to thethird through fifth embodiments below.

The advantage of performing the nitridation after the planarization ofthe electrodes 1 as shown in FIGS. 1C and 1D is that the subsequentinsulating layer 9 will not be deposited onto a tungsten surface. If theinsulating layer is silicon oxide, then it may not provide an idealadhesion to tungsten. However, silicon oxide adheres better to a metalnitride barrier, such as a tungsten nitride barrier 5.

If the plasma deposition reactor has the necessary gases plumbed, thenthe plasma nitridation can be performed in the same chamber as theinsulating layer 9 deposition, without adding any process steps. In sucha process, the nitriding plasma, such as a nitrogen or ammonia plasma,is turned on for a time to nitride the tungsten electrode 1 surfaces.Then, the nitrogen containing plasma is pumped from the depositionchamber and the insulating layer 9 deposition process begins byproviding desired precursors, such as silicon and oxygen containingprecursors (for example silane in combination with oxygen or nitrousoxide) to the deposition chamber to deposit layer 9. Preferably, layer 9is silicon oxide deposited by PECVD.

The advantage of performing the nitridation after forming the openings11 is that if the tungsten electrode sidewalls 2 are exposed in theopening 11 overetch, then the sidewalls 2 will also be nitrided, asshown in FIG. 2B. This can happen if the insulating layer 9 opening 11overetch also removes the TiN adhesion layer which may be located belowthe tungsten electrodes 1. In other words, the plurality of openings 11in the insulating layer 9 may be partially misaligned with the pluralityof the tungsten electrodes 1 and the etching step using to form theplurality of openings 11 exposes at least portions of sidewalls 2 of thetungsten electrodes 1 due to the misalignment and over etching, as shownin FIG. 2A. Then, the step of nitriding forms tungsten nitride barriers5 on the upper surfaces of electrodes 1 and tungsten nitride barriers 6on exposed portions of the sidewalls 2 of the tungsten electrodes 1 asshown in FIG. 2B.

In case misalignment occurred during formation of the openings 11, thesilicon layer 13 may extend into the overetched portions of the openings11. However, silicon layer 13 contacts only the tungsten nitridebarriers 5 and 6, but does not contact the tungsten electrodes 1directly, as shown in FIG. 2C. When the final device, such as a pillarshaped diode, is completed, it is partially misaligned with the tungstenelectrode 1 and the tungsten nitride barriers 5, 6 are located on anupper surface of the tungsten electrode and on at least a portion of asidewall of the tungsten electrode. The oxide insulating layer 9 wouldbe located around the diode, as will be described in more detail below,such that a portion 14 of the oxide insulating layer 9 located adjacentto at least one sidewall of the pillar shaped diode is nitrided.

Both non-limiting advantages of nitridation described above (improvedinsulating layer 9 adhesion to tungsten nitride and electrode 1 sidewallbarrier 6 formation) will be achieved if the nitridation is performedbefore layer 9 deposition and after formation of the openings 11 inlayer 9. Thus, if desired, the electrode 1 nitridation can be performedboth after the bottom electrode planarization as shown in FIGS. 1C and1D and after formation of the openings 11, as shown in FIG. 2B.

In the second embodiment, the conductive barriers 5 are formed by aselective deposition on exposed upper surfaces of the tungstenelectrodes 1. For example, in one aspect of the second embodiment, metalor metal alloy barriers 5 are formed by selective atomic layerdeposition on the plurality of tungsten electrodes. The barrier 5 metalor metal alloy may comprise tantalum, niobium or alloys thereof.Selective atomic layer deposition of a barrier metal, such as tantalumor niobium, is described in U.S. published patent Application Number2004/0137721 which is incorporated herein by reference in its entirety.The atomic layer deposition of the barrier 5 is preferably conductedbefore the deposition of the insulating layer 9, as shown in FIGS. 1Cand 1D. The selective deposition forms barriers 5 selectively only onthe electrodes 1 but not the adjacent insulating layer or material 3.Thus, a metallic connection from the barriers 5 of the electrodes to thetop surface of the insulating layer 9 is prevented.

In an alternative method of the second embodiment, the conductivebarriers are formed by selective plating of a barrier metal or metalalloy on the plurality of tungsten electrodes. The plating may compriseelectroless plating or electroplating which selectively plates thebarriers 5 onto the electrodes 1 but not on the adjacent insulatinglayers 3 or 9. The barrier metals or metal alloys may comprise anyconductive barrier materials that can be selectively plated onto theelectrodes and not the insulating layers from a plating solution, suchas cobalt and cobalt tungsten alloys, including CoWP. Selectivedeposition of a barrier metal alloy, such as CoWP by plating isdescribed in “Thermal Oxidation of Ni and Co Alloys Formed byElectroless Plating”, Jeff Gamindo and coauthors, MRS Abstract numberF5.9, Apr. 17-21 2006, San Francisco, incorporated herein by referencein its entirety. The selective plating may be conducted before thedeposition of the insulating layer 9 and/or through the openings 11 inthe insulating layer 9. In other words, the plating of the conductivebarriers may be conducted before the step of forming the insulatinglayer 9, such that the insulating layer 9 is formed on the plurality ofconductive barriers 5 followed by forming the plurality of openings 11in the insulating layer 9 to expose upper surfaces of the plurality ofconductive barriers 5. Alternatively, the plating of the conductivebarriers may be conducted after the step of forming the plurality ofopenings 11 in the insulating layer 9 such that the plurality ofconductive barriers are selectively formed on the upper surfaces of theplurality of tungsten electrodes 1 through the plurality of openings 11in the insulating layer 9.

As described above with respect to FIGS. 2A to 2C, the openings 11 inthe insulating layer 9 may be partially misaligned with the plurality ofthe tungsten electrodes 1, such that the step of forming the pluralityof openings 11 exposes at least portions of sidewalls 2 of the tungstenelectrodes 1. The selective deposition of the conductive barriers 5,such as the selective plating, forms the conductive barriers 5 on theupper surfaces and conductive barriers 6 on exposed portions of thesidewalls 2 of the plurality of tungsten electrodes 1.

A method according to the third embodiment forms pillar shaped devices,such as a pillar diodes, in the openings 11 in the insulating layer 9 bya modified process, as shown in FIGS. 3A-3E. The devices may be formedon the barrier layers 5, 6 of the first or second embodiments.Alternatively, the barrier layers 5, 6 may be omitted or the barriers 5may be formed by non-selective layer deposition followed byphotolithographic patterning rather than being formed by the methods ofthe first or the second embodiment.

As shown in FIG. 3A, the insulating layer 9 containing a plurality ofopenings 11 is provided over a substrate. The substrate can be anysemiconducting substrate known in the art, such as monocrystallinesilicon, IV-IV compounds such as silicon-germanium orsilicon-germanium-carbon, III-V compounds, II-VI compounds, epitaxiallayers over such substrates, or any other semiconducting ornon-semiconducting material, such as glass, plastic, metal or ceramicsubstrate. The substrate may include integrated circuits fabricatedthereon, such as driver circuits for a memory device. As described abovewith respect to the first and the second embodiments, the lowerelectrodes, such as rail shaped tungsten electrodes 1 covered withbarriers 5 are formed over the substrate as a first step in fabricatinga nonvolatile memory array. Other conductive materials, such asaluminum, tantalum, titanium, copper, cobalt, or alloys thereof, mayalso be used. An adhesion layer, such as a TiN adhesion layer may beincluded below the electrodes 1 to help the electrodes to adhere toinsulating layer 3 or other materials below the electrodes 1.

The insulating layer 9 can be any electrically insulating material, suchas silicon oxide, silicon nitride, or silicon oxynitride, or an organicor inorganic high dielectric constant material. If desired, theinsulating layer 9 may be deposited as two or more separate sublayers.Layer 9 may be deposited by PECVD or any other suitable depositionmethod. Layer 9 may have any suitable thickness, such as about 200 nm toabout 500 nm for example.

The insulating layer 9 is then photolithographically patterned to formopenings 11 extending to and exposing the upper surface of the barriers5 of the electrodes 1. The openings 11 should have about the same pitchand about the same width as the electrodes 1 below, such that eachsubsequently formed semiconductor pillar is formed on top of arespective electrode 1. Some misalignment can be tolerated, as describedabove. Preferably, the openings 11 in the insulating layer 9 have a halfpitch of 45 nm or less, such as 10 nm to 32 nm. The openings 11 with thesmall pitch may be formed by forming a positive photoresist over theinsulating layer 9, exposing the photoresist to radiation, such as 193nm radiation, while using an attenuated phase shift mask, patterning theexposed photoresist, and etching the openings 11 in the insulating layer9 using the patterned photoresist as a mask. The photoresist pattern isthen removed. Any other suitable lithography or patterning method mayalso be used. For example, other radiation wavelengths, such as the 248nm wavelength, may be used with or without the phase shift mask. Forexample, 120-150 nm, such as about 130 nm wide openings may be formedwith 248 nm lithography and 45-100 nm, such as about 80 nm wide openingsmay be formed with 193 nm lithography. Furthermore, various hardmasksand antireflective layers may also be used in the lithography, such as aBARC or DARC in combination with an insulating hardmask for 248 nmlithography, and BARC or DARC in combination with a dual W/insulatinghardmask for 193 nm lithography.

A first semiconductor layer 13 is formed in the plurality of openings 11in the insulating layer 9 and over the insulating layer 9. Thesemiconductor layer 13 may comprise silicon, germanium,silicon-germanium or a compound semiconductor material, such as a III-Vor II-VI material. The semiconductor layer 13 may be an amorphous orpolycrystalline material, such as polysilicon. The amorphoussemiconductor material may be crystallized in a subsequent step. Layer13 is preferably heavily doped with a first conductivity type dopant,such as p-type or n-type dopant, such as doped with a dopantconcentration of 10¹⁸ to 10²¹ cm⁻³. For illustration, it will be assumedthat layer 13 is a conformally deposited n-type doped polysilicon. Thepolysilicon can be deposited and then doped, but is preferably doped insitu by flowing a dopant containing gas providing n-type dopant atoms,for example phosphorus or arsenic (i.e., in the form of phosphine orarsine gas added to the silane gas) during LPCVD deposition of thepolysilicon layer. The resulting structure is shown in FIG. 3A.

As shown in FIG. 3B, an upper portion of the semiconductor layer 13,such as a polysilicon layer, is removed. The lower n-type portions 17 ofthe polysilicon layer 13 remain in lower portions of the openings 11 inthe insulating layer 9, while upper portions 19 of the plurality ofopenings 11 in the insulating layer 9 remain unfilled. N-type portions17 may be between about 5 nm and about 80 nm thick, such as about 10 nmto about 50 nm thick. Other suitable thicknesses may be used instead.

Any suitable method may be used to remove layer 13 from upper portions19 of the openings 11. For example, a two step process may be used.First, the polysilicon layer 13 is planarized with an upper surface ofthe insulating layer 9. The planarization may be performed by CMP oretchback (such as isotropic etching) with optical end point detection.Once the polysilicon layer 13 is planarized with the upper surface ofthe insulating layer 9 (i.e., such that the polysilicon layer 13 fillsthe openings 11 but is not located over the top surface of theinsulating layer 9), a second recess etching step may be performed torecess the layer 13 in the openings 11, such that only portions 17 oflayer 13 remain in the openings 11. Any selective etching step, such asa wet or dry, isotropic or anisotropic etching step which selectively orpreferentially etches polysilicon remaining in the upper portions ofopenings 11 over the insulating material of layer 9 (such as siliconoxide) may be used. Preferably, a dry etching step which provides acontrollable etch end point is used.

For example, as shown in a micrograph in FIG. 3F, the recess etchingstep is a selective dry anisotropic etching step. In this step, thefirst semiconductor layer 13 remaining in the upper portions of theplurality of openings 11 is etched with a level etch front to recess thefirst semiconductor layer 13. The level etch front provides thatportions 17 of the first semiconductor layer 13 remaining in theplurality of openings 11 have a substantially planar upper surface, asshown in FIG. 3F. This allows formation of a “parfait” shaped diode inwhich the boundary between different conductivity type regions issubstantially planar.

Alternatively, as shown in a micrograph in FIG. 3G, a selectiveisotropic etch may be used to recess layer 13. In this case, theportions of the first semiconductor layer 13 remaining in the pluralityof openings 11 have an annular (i.e., hollow ring) shape with a groovein a middle, as shown in FIG. 3G.

As shown in FIG. 3C, a second semiconductor layer 21 is then formed inthe upper portions 19 of the plurality of openings 11 in the insulatinglayer 9 and over the insulating layer 9. The second semiconductor layer21 may comprise the same or different semiconductor material as thematerial of the first semiconductor layer 13. For example, layer 21 mayalso comprise polysilicon. It may be advantageous to deposit a layer 21with a different semiconductor composition compared to the compositionof layer 13, as described in U.S. Pat. No. 7,224,013 to Herner andWalker titled “Junction diode comprising varying semiconductorcompositions” and which is incorporated by reference herein in itsentirety. For example, layer 13 may comprise silicon orsilicon-germanium alloy having a relatively low percentage of germanium,while layer 21 may comprise germanium or a silicon-germanium alloyhaving a higher percentage germanium than layer 13 or vice-versa. If ap-n type diode is being formed in the openings 11, then layer 21 may beheavily doped with opposite conductivity type dopants, such as p-typedopants, from the conductivity type of layer 13. If desired, the secondsemiconductor layer 21 have the same conductivity type as the firstlayer 13, but a lower doping concentration than layer 13.

If a p-i-n type diode is being formed in the openings 11, then thesecond semiconductor layer 21 may be an intrinsic semiconductormaterial, such as intrinsic polysilicon. In this discussion, a region ofsemiconductor material which is not intentionally doped is described asan intrinsic region. It will be understood by those skilled in the art,however, that an intrinsic region may in fact include a lowconcentration of p-type or n-type dopants. Dopants may diffuse into theintrinsic region from adjacent regions, or may be present in thedeposition chamber during deposition due to contamination from anearlier deposition. It will further be understood that depositedintrinsic semiconductor material (such as silicon) may include defectswhich cause it to behave as if slightly n-doped. Use of the term“intrinsic” to describe silicon, germanium, a silicon-germanium alloy,or some other semiconductor material is not meant to imply that thisregion contains no dopants whatsoever, nor that such a region isperfectly electrically neutral. The second semiconductor layer 21 isthen planarized at least with an upper surface of the insulating layer 9using chemical mechanical polishing to remove a first portion of thesecond semiconductor layer 21 located over the insulating layer 9 whileleaving portions 23 of layer 21 in the upper portions 19 of openings 11.Alternatively, etchback may also be used. The intrinsic region orportions 23 may be between about 110 and about 330 nm, such as about 200nm thick. The resulting device is shown in FIG. 3D.

Then, dopants of the opposite conductivity type to the conductivity typeof regions 17 are implanted into upper sections of the second portions23 of the second semiconductor layer 21 to form p-i-n pillar shapeddiodes. For example, p-type dopants are implanted into the uppersections of intrinsic portions 23 to form p-type regions 25. The p-typedopant is preferably boron which is implanted as boron or BF₂ ions.Alternatively, region 25 may be selectively deposited on region 23(after region 23 is recessed in openings 11) and then planarized ratherthan being implanted into region 23. For example, region 25 may beformed by depositing an in-situ p-type doped semiconductor layer by CVDfollowed by planarization of this layer. Region 25 may be about 10 nm toabout 50 nm thick, for example. The pillar shaped p-i-n diodes 27located in openings 11 comprise n-type regions 17, intrinsic regions 23and p-type regions 25, as shown in FIG. 3E. In general, the pillardiodes 27 preferably have a substantially cylindrical shape with acircular or roughly circular cross section having a diameter of 250 nmor less. Alternatively, pillar diodes with polygonal cross sectionalshapes, such as rectangular or square shapes may also be formed byforming openings 11 with polygonal cross sectional shapes instead ofcircular or oval cross sectional shapes.

Optionally, n+ dopant diffusion is prevented during subsequent intrinsicsilicon deposition by the method described in U.S. Published Application2006/0087005 titled “Deposited semiconductor structure to minimizeN-type dopant diffusion and method of making” which is incorporatedherein by reference in its entirety. In this method, the n-typesemiconductor layer, such as an n-type polysilicon or amorphous siliconlayer, is capped by a silicon-germanium capping layer having at least 10atomic percent germanium. The capping layer may be about 10 to about 20nm thick, preferably no more than about 50 nm thick, and contains littleor no n-type dopant (i.e., the capping layer is preferably a thin,intrinsic silicon-germanium layer). The intrinsic layer of the diode,such as a silicon layer or silicon-germanium layer having less than 10atomic percent germanium is deposited on the capping layer.Alternatively, an optional silicon rich oxide (SRO) layer is formedbetween the n-type region 17 and the intrinsic region 23 of each diode27. The SRO region forms a barrier that prevents or decreases phosphorusdiffusion from bottom n-type region 17 of the diode into the undopedregion 23.

In the illustrative example, the bottom region 17 of the diode 27 is N⁺(heavily doped n-type), and the top region 25 is P⁺. However, thevertical pillar can also comprise other structures. For example, bottomregion 17 can be P⁺ with N⁺ top region 25. In addition, the middleregion can intentionally be lightly doped, or it can be intrinsic, ornot intentionally doped. An undoped region will never be perfectlyelectrically neutral, and will always have defects or contaminants thatcause it to behave as if slightly n-doped or p-doped. Such a diode canbe considered a p-i-n diode. Thus, a P₊/N⁻/N⁺, P⁺/P⁻/N⁺, N⁺/N⁻/P⁺ orN⁺/P⁻/P⁺ diode can be formed.

Turning to FIG. 4, upper electrodes 29 can be formed in the same manneras the bottom electrodes 1, for example by depositing an adhesion layer,preferably of titanium nitride, and a conductive layer, preferably oftungsten. Conductive layer and adhesion layer are then patterned andetched using any suitable masking and etching technique to formsubstantially parallel, substantially coplanar conductor rails 29,extending perpendicular to conductor rails 1. In a preferred embodiment,a photoresist is deposited, patterned by photolithography, theconductive layers are etched, and then the photoresist is removed usingstandard process techniques. Alternatively, an optional insulatingoxide, nitride, or oxynitride layer may be formed on heavily dopedregions 25, and the conductors 29 are formed by a Damascene process, asdescribed in Radigan et al., U.S. patent application Ser. No.11/444,936, “Conductive Hard Mask to Protect Patterned Features DuringTrench Etch,” filed May 31, 2006, hereby incorporated by reference inits entirety. Rails 29 may be about 200 nm to about 400 nm thick.

Next, another insulating layer (not shown for clarity) is deposited overand between conductor rails 29. The insulating material can be any knownelectrically insulating material, such as silicon oxide, siliconnitride, or silicon oxynitride. In a preferred embodiment, silicon oxideis used as this insulating material. This insulating layer can beplanarized with the upper surface of the conductor rails 29 by CMP oretchback. A three dimensional view of the resulting device is shown inFIG. 4.

The pillar device, such as a diode device, may comprise a one-timeprogrammable (OTP) or re-writable nonvolatile memory device. Forexample, each diode pillar 27 may act as a steering element of a memorycell and another material or layer 31 which acts as a resistivityswitching material (i.e., which stores the data) is provided in serieswith the diode 27 between the electrodes 1 and 29, as shown in FIG. 4Specifically, FIG. 4 shows one nonvolatile memory cell which comprisesthe pillar diode 27 in series with the resistivity switching material31, such as an antifuse (i.e., antifuse dielectric), fuse, polysiliconmemory effect material, metal oxide (such as nickel oxide, perovskitematerials, etc), carbon nanotubes, phase change materials, switchablecomplex metal oxides, conductive bridge elements, or switchablepolymers. The resistivity switching material 31, such as a thin siliconoxide antifuse dielectric layer, may be deposited over the diode pillar27 followed by the deposition of the upper electrode 29 on the antifusedielectric layer. Antifuse dielectric 31 may also be formed by oxidizingan upper surface of the diode 27 to form a 1 to 10 nm thick siliconoxide layer. Alternatively, the resistivity switching material 31 may belocated below the diode pillar 27, such as between the barrier 5 andanother conductive layer, such as TiN layer. In this embodiment, aresistivity of the resistivity switching material 31 is increased ordecreased in response to a forward and/or reverse bias provided betweenthe electrodes 1 and 29.

In another embodiment, the pillar diode 27 itself may be used as thedata storage device. In this embodiment, the resistivity of the pillardiode is varied by the application of a forward and/or reverse biasprovided between the electrodes 1 and 29, as described in U.S. patentapplication Ser. No. 10/955,549 filed Sep. 29, 2004 (which correspondsto US Published Application 2005/0052915 A1) and U.S. patent applicationSer. No. 11/693,845 filed Mar. 30, 2007 (which corresponds to USPublished Application 2007/0164309 A1), both of which are incorporatedby reference in their entirety. In this embodiment, the resistivityswitching material 31 may be omitted if desired. While a nonvolatilememory device has been described, other devices, such as other volatileor nonvolatile memory devices, logic devices, display devices, lightingdevices, detectors, etc., may also be formed by the methods describedabove. Furthermore, while the pillar shaped device was described asbeing a diode, other similar pillar shaped devices, such as transistorsmay also be formed.

Formation of a first memory level has been described. Additional memorylevels can be formed above this first memory level to form a monolithicthree dimensional memory array. In some embodiments, conductors can beshared between memory levels; i.e. top conductor 29 would serve as thebottom conductor of the next memory level. In other embodiments, aninterlevel dielectric (not shown) is formed above the first memorylevel, its surface planarized, and construction of a second memory levelbegins on this planarized interlevel dielectric, with no sharedconductors.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as a wafer, withno intervening substrates. The layers forming one memory level aredeposited or grown directly over the layers of an existing level orlevels. In contrast, stacked memories have been constructed by formingmemory levels on separate substrates and adhering the memory levels atopeach other, as in Leedy, U.S. Pat. No. 5,915,167, “Three dimensionalstructure memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

A monolithic three dimensional memory array formed above a substratecomprises at least a first memory level formed at a first height abovethe substrate and a second memory level formed at a second heightdifferent from the first height. Three, four, eight, or indeed anynumber of memory levels can be formed above the substrate in such amultilevel array.

In a fourth embodiment of the invention, alternative etching and dopingsteps are used to form the pillar shaped device, such as a diode 27. Inthis embodiment, etch selectivity of various conductivity types ofpolysilicon is used in the recess etching step to provide end pointdetection. Specifically, phosphorus doped polysilicon has a faster etchrate than undoped silicon (seehttp://www.clarycon.com/Resources/Slide3t.jpg andhttp://www.clarycon.com/Resources/Slide5i.jpg for data showing thatdifferently doped polysilicon has different etch rates). The etchingrates from the above mentioned website for phosphorus doped, boron dopedand undoped polysilicon are shown in FIG. 5A.

The depth of the high-etch rate n-type doped layer can be tailored withthe implant dose and energy. One optical etch endpoint detection methodinvolves monitoring for a change in intensity of a wavelength that ischaracteristic to particular reactant or product in the etchingreaction. When the etching endpoint is achieved, there will be a lowerdensity of etch reaction products in the plasma, so the endpoint can betriggered, stopping the etch. Another etch endpoint detection uses amass spectrometer to monitor for a particular species in the exhauststream from the dry etching reaction, called RGA (residual gasanalysis). The mass spectrometer can be located near or in the exhaustconduit of the etching reaction chamber. In this case, the RGA monitorsfor a phosphorus containing species in the exhaust stream, and providesan endpoint sign or trigger on a drop in the signal.

In the method of the fourth embodiment, the first polysilicon layer 13is deposited undoped (i.e., intrinsic), as shown in FIG. 5B. Layer 13 isthen implanted with phosphorus to a predetermined depth before or afterlayer 13 is planarized with the upper surface of the insulating layer 9to form an implanted region 101, as shown in FIG. 5C. The depth of theimplant is selected such that the bottom 103 of the phosphorus implantedregion 101 will be located at or around the upper surface of region 17that was shown in FIG. 3B. Intrinsic portions 105 of the firstsemiconductor layer 13 remain in lower portions of the plurality ofopenings 11.

The first polysilicon layer 13 is then selectively etched, such as byusing anisotropic plasma etching (using for example SF₆, CF₄, HBr/Cl₂ orHBr/O₂ plasma) to recess layer 13 in the openings 11. The phosphorusdoped region 101 of the first polysilicon layer 13 is etched until theintrinsic portions 105 of the first polysilicon layer are reached, asshown in FIG. 5D. In other words, once the bottom 103 of the phosphorusimplanted region 101 is reached during the etching step (and thus theintrinsic portions 105 of the first polysilicon layer 13 are reachedduring the etching step), as detected optically or by RGA, the etchingis stopped. Specifically, when the bottom 103 of the phosphorus dopedregion 101 is reached, the intensity of the phosphorus characteristicwavelength will decrease in optical endpoint detection or the amount ofphosphorus containing species detected by RGA will decrease. Theremaining intrinsic portions 105 of layer 13 in openings 11 are thenredoped with n-type dopant, such as by implanting phosphorus or arsenicinto portions 105 to form n-type portions 17, as shown in FIG. 5E. Thesecond semiconductor layer, such as the intrinsic semiconductor layer 21is then deposited onto portions 17 as shown in FIG. 3C and the processcontinues as in the third embodiment. To form a diode 27 with a p-typebottom region, the portions 105 are implanted with boron or BF₂ afterthe recess etching. Furthermore, rather than using phosphorus implantedregion for end point detection, boron or BF₂ implanted regions may beused, and a characteristic boron wavelength or RGA signature ismonitored instead.

Furthermore, optical endpoint detection can be used to determine whenlayer 13 is planarized with the upper surface of the insulating layer 9.Once layer 13 is planarized, the upper surface of the insulating layer 9is exposed. Thus, the optical signature of the surface will change froma polysilicon signature to a signature characteristic of presence ofboth polysilicon and insulator (such as silicon oxide).

In a fifth embodiment of the present invention, a sacrificial layer isused to form the pillar shaped devices. FIGS. 6A-6G illustrate the stepsin the method of the fifth embodiment.

First, a plurality of lower electrodes 1 are formed over a substrate, asdescribed above with respect to the prior embodiments. For example,tungsten electrodes 1 with barriers 5 of the first or the secondembodiments may be provided (electrodes 1 and barriers 5 are omittedfrom FIG. 6A for clarity and are shown in the final device depicted inFIG. 6G). Then, the insulating layer 9 containing a plurality ofopenings 11 having a first width is provided over the electrodes 1 andbarriers 5 (one opening 11 is shown in FIG. 6A for clarity). An optionalhardmask layer 33 may also be formed over the insulating layer 9. Then,first semiconductor regions of a first conductivity type (such as n-typepolysilicon regions) 17 are formed on the lower electrodes. For example,the methods of the third or fourth embodiments may be used to formregions 17. Then, a sacrificial material 35 is formed in the pluralityof first openings 11. The sacrificial material may be any suitablesoluble organic material which is used in dual damascene via firstmethods. For example, Wet Gap Fill (WGF) 200 material provided by BrewerScience, Inc. may be used as sacrificial material 35. The device at thisstage in the process is shown in FIG. 6A.

Then, as shown in FIG. 6B, an optional antireflective layer 37, such asa BARC layer 37 m is formed over the insulating layer 9 and over theoptional hardmask 33. A photoresist layer 39 is then exposed andpatterned over the BARC layer 37. The device at this stage in theprocess is shown in FIG. 6B.

As shown in FIG. 6C, the patterned photoresist is then used as a mask toetch a plurality of second openings 41 (one opening 41 is shown in FIG.6C for clarity) in the insulating layer 9 to expose the sacrificialmaterial 35 in openings 11. The second openings 41 are wider than thefirst openings 11. A portion of the sacrificial material 35 may beetched during the formation of the second openings. The second openings41 comprise trench shaped openings in which the sacrificial material isexposed in a portion of the bottom of the trench.

As shown in FIG. 6D, the sacrificial material is selectively removedfrom the first openings 11 through the second openings 41. Any suitableliquid etching material or developer may be used to remove material 35from openings 11 to expose n-type polysilicon regions 17 in the openings11.

Then, as shown in FIG. 6E, second semiconductor regions of a secondconductivity type are formed in the first openings 11. For example, theintrinsic polysilicon layer 21 may be formed in openings 11 and 41 andover the insulating layer 9.

The polysilicon layer 21 is then planarized and recessed using themethods described in the third embodiment. Preferably, the remainingportion 23 of polysilicon layer 21 is recessed such that its uppersurface is level with the top of the openings 11 (i.e., the top ofportion 23 is level with the bottom of trench 41). P-type regions 25 arethen implanted into intrinsic regions 23 as described in the thirdembodiment above. The device at this stage is shown in FIG. 6F. Regions17, 23 and 25 form pillar shaped diodes 27 in the first openings 11.

Then, as shown in FIG. 6G, upper electrodes are formed in the trenches41 in the insulating layer 9 by a damascene process, such that the upperelectrodes contact the p-type semiconductor regions 25 of the diodes 27.The upper electrodes may comprise a TiN adhesion layer 43 and tungstenconductors 29. The upper electrodes are then planarized by CMP oretchback with the upper surface of the insulating layer 9. If desired, alower TiN adhesion layer 45 may also be formed below the lowerelectrodes 1. The trench may be about 200 nm to about 400 nm deep andthe diode 27 may about 200 nm to about 400 nm high, such as about 250 nmhigh.

The pillar shaped devices may be made using any one or more stepsdescribed above with respect to any one or more of the first throughfifth embodiments. Dependent on the process steps used, the completeddevice may have one or more of the following features shown in FIGS. 7Aand 7B.

For example, as shown in FIG. 7A, the n-type region 17 of the diode 27may contain a first vertical seam 47, while the p-type region 25 (aswell as the intrinsic region 23) of the diode 27 may contain a secondvertical seam 49. The seams 47, 49 may be formed if the deposition ofthe polysilicon layers 13 and 21 does not completely fill the openings11 during the separate deposition steps. The first 47 and the second 49vertical seams do not contact each other. The seams do not contact eachother because the polysilicon layers 13 and 21 are deposited in separatesteps as shown in FIGS. 3A-3E. Specifically, without wishing to be boundby a particular theory, it is believed that the bottom portion of layer21 which contacts region 17 would not form the seam since the bottomportion of layer 21 may fill the opening 11 completely. However,depending on the deposition process of the polysilicon layers 13 and 21the seams may be omitted.

Furthermore, as also shown in FIG. 7A, the sidewalls 51 of the firstconductivity type region (such as the n-type region 17) may have adifferent taper angle than sidewalls 53 of the second conductivity typeregion (such as the p-type region 25 and/or intrinsic region 23) of thediode. A discontinuity 55 is located in a sidewall of the diode 27 wherethe differently tapered sidewalls 51, 53 meet. Specifically, the firstconductivity type region 17 has a narrower taper angle than the secondconductivity type region 25 and the discontinuity 55 is a step in thesidewall of the diode between the intrinsic semiconductor region 23 andthe n-type conductivity type region 17. Without wishing to be bound by aparticular theory, it is believed that the different tapers and thediscontinuity may be formed because the recess etchback of layer 13shown in FIG. 3B is more isotropic than the step of etching the openings11 in the insulating layer 9 shown in FIG. 3A. Thus, during the etchbackof layer 13, the upper portions 19 of openings 11 are also etched andare widened compared to lower portions of openings 11. Thus, layers 13and 21 which fill the lower and upper portions of openings 11,respectively, assume the different tapers of the respective portions ofthe openings. The different tapers and the discontinuity may be avoidedif the recess etching step of layer 13 is conducted without widening theupper portions 19 of the openings.

If the barriers 5 are formed by nitriding the electrodes 1 through theopenings 11 in the insulating layer 9, as shown in FIG. 2B, then theportion of the insulating layer 9 located adjacent to at least onesidewall of the pillar shaped diode 27 is nitrided. For example, asshown in FIGS. 2B and 7A, if layer 9 is silicon oxide, then a nitridedoxide, such as silicon oxynitride or nitrogen containing silicon oxideregion 14 is formed on the sidewalls 12 of the openings 11 around thediode 27. Furthermore, if the upper portion of the insulating layer 9adjacent to the p-type region 25 of the diode contains a boron gradient,then it indicates that boron was implanted into the insulating layer 9in addition to being implanted into upper portions of regions 23 to formregions 25, as shown in FIGS. 3E and 7A.

FIG. 7B shows an inset portion in FIG. 7A around the barriers 5, 6. Ifthe pillar shaped diode is partially misaligned with the tungstenelectrode, as shown in FIGS. 2A, 2B and 7B, then the tungsten nitridebarrier 5 is located on an upper surface of the tungsten electrode 1 andthe tungsten nitride barrier 6 is located on at least a portion of asidewall of the tungsten electrode 1, as shown in FIG. 7B. Furthermore,if the barrier 5 is formed by nitriding the tungsten electrodes 1 beforeforming the insulating layer 9, as shown in FIGS. 1C and 1D, then a thinnitrogen rich region, such as a 1-10 nm thick nitrogen rich region 7 isformed on top of the lower insulating layer or material 3. For example,if layer 3 comprises an oxide, such as silicon oxide, then its topportion 7 is nitrided to form silicon oxynitride or nitrogen containingsilicon oxide.

Based upon the teachings of this disclosure, it is expected that one ofordinary skill in the art will be readily able to practice the presentinvention. The descriptions of the various embodiments provided hereinare believed to provide ample insight and details of the presentinvention to enable one of ordinary skill to practice the invention.Although certain supporting circuits and fabrication steps are notspecifically described, such circuits and protocols are well known, andno particular advantage is afforded by specific variations of such stepsin the context of practicing this invention. Moreover, it is believedthat one of ordinary skill in the art, equipped with the teaching ofthis disclosure, will be able to carry out the invention without undueexperimentation.

The foregoing details description has described only a few of the manypossible implementations of the present invention. For this reason, thisdetailed description is intended by way of illustration, and not by wayof limitations. Variations and modifications of the embodimentsdisclosed herein may be made based on the description set forth herein,without departing from the scope and spirit of the invention. It is onlythe following claims, including all equivalents, that are intended todefine the scope of this invention.

1. A method of making a semiconductor device, comprising: forming aplurality of lower electrodes over a substrate; forming an insulatinglayer containing a plurality of first openings having a first width,such that the lower electrodes are exposed in the first openings;forming first semiconductor regions of a first conductivity type in thefirst openings; forming a sacrificial material in the plurality of firstopenings over the first semiconductor regions; forming a plurality ofsecond openings in the insulating layer to expose the sacrificialmaterial, the second openings having a second width greater than thefirst width; removing the sacrificial material from the first openingsthrough the second openings; forming second semiconductor regions of asecond conductivity type in the first openings, wherein the first andthe second semiconductor regions form pillar shaped diodes in the firstopenings; and forming upper electrodes in the second openings in theinsulating layer such that the upper electrodes contact the secondsemiconductor regions.
 2. The method of claim 1, further comprisingforming intrinsic third semiconductor regions between the first and thesecond semiconductor regions to form p-i-n pillar shaped diodes.
 3. Themethod of claim 2, wherein: the step of forming the first semiconductorregions comprises forming a first semiconductor layer in the pluralityof first openings in the insulating layer and over the insulating layerfollowed by removing a portion of the first semiconductor layer, suchthat the first semiconductor regions remain in lower portions of theplurality of first openings and upper portions of the first plurality ofopenings remain unfilled; and the step of forming the secondsemiconductor regions comprises forming a second semiconductor layer inthe upper portions of the plurality of first openings in the insulatinglayer and over the insulating layer followed by removing a portion ofthe second semiconductor layer located over the insulating layer, suchthat the second semiconductor regions remain in the upper portions ofthe plurality of first openings in the insulating layer.